The complement system is a component of the innate and adaptive immune system (reviewed by Volanakis, J. E., 1998. Chapter 2. In The Human Complement System in Health and Disease. Edited by J. E. Volanakis, and M. M. Frank. Marcel Dekker, Inc., New York pp 9-32). Complement plays an important role in microbial killing, and for the transport and clearance of immune complexes. Many of the activation products of the complement system are also associated with proinflammatory or immunoregulatory functions. The complement system consists of plasma and membrane-associated proteins that are organized in three enzymatic-activation cascades: the classical, the lectin, and the alternative pathways (FIG. 1). All three pathways can lead to the formation of the terminal complement complex (TCC) and an array of biologically active products.
In some cases, complement activation is initiated either by specific antibodies recognizing and binding to a variety of pathogens and foreign molecules, and/or by direct interaction of complement proteins with foreign substances. On activation, these pathways result in the formation of protease complexes, the C3-convertases. The classical pathway C3-convertase, C4b2a, and the alternative pathway C3-convertase, C3bBb, are both able to cleave the α chain of C3 generating C3b. C3b has the potential to bind covalently to biological surfaces. C3b binding leads to opsonization for phagocytosis by polymorphonuclear cells and macrophages. When additional C3b is available, the C3-convertases can function as C5-convertases, cleaving C5 and initiating the assembly of the TCC, or membrane attack complex (MAC), which mediates cellular lysis by insertion of pore-forming protein complexes into targeted cell membranes.
In the classical pathway as shown in FIG. 1A, C1q, a collagenous subcomponent of the first component (C1), binds to immunoglobulins within immune complexes, and its associated serine proteases, C1r and C1s, become activated. This complement cascade is initiated by the subsequent cleavage of C4 and C2, followed by C3 activation. The resulting C3b fragment not only acts as an opsonin but also leads to the membrane attack complex (MAC) formation in the lytic pathway. In innate immunity, a complex composed of a recognition molecule (lectin) and serine proteases, termed the mannose-binding lectin (MBL)-associated serine protease (MASP), activates C4 and C2 upon binding to carbohydrates on the surface of microorganisms via the lectin pathway. This binding occurs in the absence of immunoglobulins. Recognition molecules of the lectin pathway found in jawed vertebrates are MBLs and ficolins, both of which are characterized by the presence of a collagen-like domain, like C1q, and a carbohydrate binding domain having a common binding specificity for GlcNAc. MASPs and C1r/C1s share the same domain organization and form a subfamily of serine proteases.
The lectin complement pathway in innate immunity is closely related to the classical complement pathway in adaptive immunity, e.g., with respect to the structures and functions of their components. Both pathways are typically initiated by complexes consisting of collagenous proteins and serine proteases of the mannose-binding lectin (MBL)-associated serine protease (MASP)/C1r/C1s family. It has been speculated that the classical pathway emerged evolutionarily after the lectin pathway.
Activation of the alternative complement pathway, shown in FIG. 1B, typically begins when C3b protein (or C3i) binds to a cell and other surface components, e.g., of microbes. C3b can also bind to immunoglobulin G (IgG) antibodies. Alternative pathway Factor B protein then combines with the C3b protein to form C3bB. Factor D protein then splits the bound Factor B protein into fragments Bb and Ba, forming C3bBb. Properdin then binds to the Bb to form C3bBbP that functions as a C3 convertase capable of enzymatically splitting typically hundreds of molecules of C3 into C3a and C3b. Some of the C3b subsequently binds to some of the C3bBb to form C3bBbC3b, a C5 convertase capable of splitting molecules of C5 into C5a and C5b.
Since C3b is free in the plasma, it can bind to either a host cell or pathogen surface. To prevent complement activation from proceeding on the host cell, there are several different kinds of regulatory proteins that disrupt the complement activation process. Complement Receptor 1 (CR1 or CD35) and DAF (also known as CD55) compete with Factor B in binding with C3b on the cell surface and can even remove Bb from an already formed C3bBb complex. The formation of a C3 convertase can also be prevented when a plasma protease called Factor I cleaves C3b into its inactive form, iC3b. Factor I works with C3b-binding protein cofactors such as CR1 and Membrane Cofactor of Proteolysis (MCP or CD46). Another complement regulatory protein is Factor H which either competes with factor B, displaces Bb from the convertase, acts as a cofactor for Factor I, or preferentially binds to C3b bound to vertebrate cells.
The precise function of the complement system depends on its regulation, as activation of the complement cascade leads to the production of a number of proteins that contribute to inflammation. This is beneficial when contributing to a host defense, but can be detrimental if activated on self tissue. Typically, activation of C3 in the blood is kept at a low level, and C3b deposition is limited to the surface of pathogens.
The human wild type complement factor B protein is a 764 amino acid, single-chain glycoprotein (approximately 93-kDa) composed of five protein domains (Mole et al., 1984 The J. Biol Chem, 259:6, 3407-3412). A human wild type complement factor B protein (fB) is typically expressed with an N-terminal 25 amino acid signal peptide, e.g., see SEQ ID NO: 1. The amino-terminal region (Ba) of human wild type complement factor B protein consists primarily of three short consensus repeats. The middle region is a type A domain similar to those found in von Willebrand factor (Colombatti et al., Blood (1991) 77(11):2305-15). The carboxy terminus is a serine protease (SP) domain (Perkins and Smith, Biochem J. (1993) 295(Pt 1):109-14; Hourcade et al., JBC (1998) 273(40):25996-6000; Hourcade et al. J Immunol. (1999) 162(5):2906-11; Xu et al., J Biol Chem. 2000 275(1):378-85; Milder et al. Nat Struct Mol Biol (2007) 14(3):224-8).
Complement factor B analogs and their use for inhibiting complement and treating complement mediated diseases are described in PCT Publication No. WO08/106644 and U.S. Patent Publication No. US20100120665. For example, the human complement factor B protein analog, hfB3 (described in U.S. Patent Publication No. 20100120665), is a dominant negative human factor B protein variant that efficiently inhibits the alternative complement (AP) activity. hfB3 protein (SEQ ID NO:4) has five amino acid changes compared to a human wild type factor B protein (SEQ ID NO:1). The five amino acid changes enable hfB3 protein to (i) bind much tighter to C3b protein, (ii) resist C3b-dependent cleavage by factor D protein, and (iii) bind tighter to factor D protein when compared to the wild type factor B protein. The tighter binding of hfB3 protein with C3b protein and factor D protein sequester two essential components of the alternative complement pathway (ACP) in an inactive C3 convertase (hfB3), blocking the AP activity. Since C3b-bound hfB3 protein cannot be cleaved by factor D protein, the conformational change of hfB3 protein does not occur and the serine protease at the C-terminus of hfB3 protein is not activated.
Both human wild type complement factor B protein and hfB3 contain 23 cysteine amino acids. The “active” forms of both have all of the cysteines forming disulfide bonds with one of the other cysteines, with the exception of the cysteine corresponding to the C292 of SEQ ID NO: 1. The C292 of the “active forms” of hfB3 and wild type factor B is a free cysteine (Parkes et al. 1983 Biochem J. 213, 201-209) and is highly conserved among various mammalian species, e.g., see Table 1, below.
Citation or discussion of a reference herein shall not be construed as an admission that such is prior art to the present invention.